1. Field of the Invention
The present invention relates to an optical data recording/reproducing apparatus, and, more particularly, to a tracking-deviation detector apparatus which detects a tracking deviation during data recording to prevent overwriting of data on an adjacent track.
2. Description of the Related Art
A disk medium is one type of recording mediums used in optical data recording/reproducing apparatuses. A disk has several sectors set in its radial direction, and has a number of tracks set along the concentric circles of the sectors. Each track has a number of pits provided, which represent data.
To accurately record data on such optical disk using a laser beam, it is necessary to control the position of an objective lens, etc. so that the laser beam is properly irradiated on a predetermined position on a track. More specifically, when the spot of the laser beam is deviated from a predetermined pit position due to eccentricity of the disk or deformation of tracks, a servo mechanism works and the objective lens is moved according to the amount of the deviation so that the beam spot is shifted to the predetermined pit position. This is generally called tracking servo control, which is accomplished by, for example, the three spot methods, the wobbling method, or the push-pull method.
Japanese patent publication No. 58-55566 discloses an example of tracking servo control in which an electrical feedback circuit is provided for controlling a motor device used for shifting a lens, the amount of the shift depending on a voltage supplied from a photoelectric converter which converts light reflected from a track into electrical signals.
In spite of the tracking servo control, the spot of the laser beam sometimes can deviate from the predetermined pit position. This is called a tracking deviation. Such tracking deviation may result in undesirable overwriting of data on an adjacent track on which data has already been recorded, thus destroying the data recorded on the adjacent track.
To cope with the above problem, first, detection of a tracking deviation is required before undesirable overwriting occurs. Detecting methods include checking the consecutiveness of address data (data already recorded in a block, on a track, in a sector, etc.). For instance, if an address should not be read in the correct order or within a predetermined time, it would be discriminated that a tracking deviation had occurred. However, the overwriting often occurs before detecting the tracking deviation. So, as a solution to this, the use of two light beams has been considered; while data record/ nonrecord is being detected with the preceding beam, data is recorded using the succeeding beam. This is, however, technically very difficult, and is not worked out yet.
Accordingly, there is a growing demand for some means with a simple structure, which can detect a tracking deviation during data recording and can prevent overwriting of data on an adjacent track upon detection of such a tracking deviation.